1. Field of the Invention
The present invention relates generally to piston machines or engines. The invention relates particularly to a hydraulically operated piston machine which has at least one cylinder and a piston axially movably guided in the cylinder. The piston is connected to a crankshaft or the like by means of a connecting rod. The connecting rod and the piston each have spherically-shaped bearing surfaces serving for connecting the connecting rod and the piston with each other in an articulated manner.
2. Description of the Prior Art
In known piston machines which operate in a similar manner as internal combustion engines, the force of the piston is transmitted to a piston pin onto the connecting rod and the crankshaft connected to the connecting rod. Since the connecting rod is pivotable only in one plane transversely of the axis of the piston pin, the eye of the connecting rod, i.e., the bore in the connecting rod in which the piston pin of the piston is mounted, must be arranged exactly parallel to the axis of the crankshaft in order to avoid deviations in the parallelism and, thus, canting or tilting of the piston within the cylinder. This exact arrangement is technically very difficult to obtain. Even slight deviations of the parallelism between the axis of the crankshaft and the axis of the connecting rod eye causes canting of the piston which leads to premature wear and failure of the machine.
In order to avoid these disadvantages, piston machines have been developed in which the connection between connecting rod and piston permits relative movement between piston and connecting rod in more than one axial direction, so that any deviations in parallelism can be compensated. A piston machine of this type has been disclosed in the magazine "asr. fur angewandte Antriebstechnik"[Asr. Digest for Applied Drive Technology], Vol. 10, October 1975, Page 28, Illustration 2. In this prior art embodiment, the piston force is transmitted onto the connecting rod by means of a spherical joint. The spherical joint is formed by a ball at the end of the connecting rod shaft which is supported in a corresponding spherical socket in the piston.
By using a spherical joint between the piston and the connecting rod, the above-mentioned deviations in parallelism are avoided, so that the overall manufacturing quality does not have to be as high. Thus, manufacturing the machine becomes simpler and less expensive. However, the above-described embodiment has the disadvantage that the entire force to be transmitted from piston to crankshaft must be absorbed by the spherical joint between the piston and the connecting rod. This leads to high surface pressures and, thus, to increased wear in the joint, particularly when, as in a hydrostatic piston machine, pressure medium is supplied between work cylinder and crankshaft through the connecting rod in order to relieve the load on the crankshaft and to lubricate the crankshaft.
In the above-described embodiment, the ball is flattened in the region of the center axis of the connecting rod, so that pressure medium can be conducted to an eccentric member of the crankshaft through a central bore in the piston and a subsequent duct in the connecting rod. The ball must have the flattened portion in order to ensure lubricant supply to the crankshaft in any position of the connecting rod. This flattened portion reduces the bearing surface of the spherical joint, so that the force is transmitted via a spherically annular contact zone. This results in high surface pressure and correspondingly high friction forces which substantially reduce the efficiency of the piston machine and increase the wear in the joint.
It is, therefore, the primary object of the present invention to provide a piston machine of the above-described type in which the disadvantages of the known piston machines are avoided. Specifically, with structurally simple means, a piston machine is to be provided in which the frictional forces and moments in the spherical joint between connecting rod and piston are reduced, in order to increase the efficiency of the machine and to reduce the wear in the joint.